Apparatus for controlling an induction motor

ABSTRACT

If a speed instruction value is less than a predetermined value, the output current of a power-conversion unit is controlled to be larger than a current value in an ordinary no-load operation, or a frequency instruction value is calculated based on a speed instruction value in place of an estimated speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of allowed application Ser. No.09/550,252, filed in the U.S. Patent and Trademark Office on Apr. 14,2000 and priority is hereby claimed under 35 USC 119 and 120 based onthe aforesaid U.S. patent application and Japanese Patent ApplicationSer. No. 11-115941, filed Apr. 23, 1999.

BACKGROUND OF THE INVENTION

[0002] The present invention relates in general to an apparatus forcontrolling the rotational speed (hereafter referred to as the speed) ofan induction motor; and, the invention relates especially to a anapparatus using a vector control method which operates without a speedsensor, and is capable of achieving a highly accurate speed controlwithout a speed sensor, and which can obtain high torque from a zerospeed range.

[0003] In the vector control of an induction motor, the output frequencyof a power-conversion unit for the induction motor usually correspondsto the sum of the speed and the calculated slip frequency. On the otherhand, in the vector control method which operates without a speedsensor, the output frequency of the power-conversion unit is controlledwith an estimated value of the speed in place of a detected value of thespeed. However, since the estimated value of the speed includes anerror, the actual slip frequency shifts from the target reference value.In this situation, the magnetic flux (hereafter referred to as the flux)in the induction motor varies according to the torque, and accordinglythe torque generated by the induction motor is not proportional to thetorque current, which in turn causes a shortage of torque in an extremecase.

[0004] Setting-errors in characteristic parameters of the inductionmotor, which are used for estimating the speed, and changes in the fluxin the induction motor, which are caused by the errors, etc., areconsidered to be the causes of the errors in the estimated speed. Ameans to effectively correct those changes of the flux has not beendevised, and a shortage of torque sometimes occurs in the range nearzero speed. A report “Simplified Vector Control System without Speed andVoltage Sensors—Effects of Setting Errors in Control Parameters andtheir Compensation” by T. Okuyama et al., T. IEE Japan, Vol. 110-D, No.5, '90, discloses the effects of the setting errors in the parametersand a means to compensate the effects due to the setting errors of theparameters of the induction motor.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide a an apparautsusing method of accurately and efficiently controlling an inductionmotor without experiencing the effects of errors in an estimated speeddue to changes in constants of the induction motor.

[0006] To achieve the above object, the present invention provides aspeed-control apparatus for an induction motor for controlling thecurrent output from a power-conversion unit so that it is larger than acurrent value in the ordinary no-load operation, or for controlling thefrequency output from the power-conversion unit by calculating afrequency instruction value based on a speed instruction value in placeof an estimated speed value if the speed instruction value is less thana predetermined value. By the above control, it is possible to prevent ashortage of torque in the low speed range near zero speed.

BRIEF DESCRIPTION OF THE DRAWINGS:

[0007]FIG. 1 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor of anembodiment according to the present invention.

[0008]FIG. 2 is a schematic diagram showing the calculational functionperformed by the speed estimator in the speed-control apparatus shown inFIG. 1.

[0009]FIG. 3 is a diagram showing the relationship between the speed andthe torque generated in an induction motor controlled by a conventionalcontrol method.

[0010]FIG. 4 is a diagram showing the relationship between the speed andthe torque generated in an induction motor controlled by an apparatususing the control method according to the present invention.

[0011]FIG. 5 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor ofanother embodiment according to the present invention.

[0012]FIG. 6 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor of stillanother embodiment according to the present invention.

[0013]FIG. 7 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor of yetanother embodiment according to the present invention.

[0014]FIG. 8 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor ofanother embodiment according to the present invention.

[0015]FIG. 9 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor ofanother embodiment according to the present invention.

[0016]FIG. 10 is a schematic diagram showing the calculational functionperformed by the slip estimator in the speed-control apparatus shown inFIG. 9.

[0017]FIG. 11 is a diagram showing the relationship between the speedand the torque generated in an induction motor controlled with the slipestimator and the slip frequency-calculator according to the presentinvention.

[0018]FIG. 12 is a schematic diagram showing the calculational functionperformed by the slip frequency-calculation unit in the speed-controlapparatus shown in FIG. 9.

[0019]FIG. 13 is a schematic diagram showing the calculational functionperformed by the d-axis flux estimator in the slip frequency-calculationunit shown in FIG. 12.

[0020]FIG. 14 is a schematic diagram showing the calculational functionperformed by the q-axis flux estimator in the slip frequency-calculationunit shown in FIG. 12.

[0021]FIG. 15 is a schematic diagram showing the calculational functionperformed by the slip estimator in the slip frequency-calculation unitshown in FIG. 12.

[0022]FIG. 16 is a schematic diagram showing the calculational functionperformed by another example of the slip frequency-calculation unit inthe speed-control apparatus shown in FIG. 9.

[0023]FIG. 17 is a schematic diagram showing the calculational functionperformed by the flux estimator in the slip frequency-calculation unitshown in FIG. 16.

[0024]FIG. 18 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor ofanother embodiment according to the present invention.

[0025]FIG. 19 is a schematic block diagram showing the circuitcomposition of a speed-control apparatus for an induction motor ofanother embodiment according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS:

[0026] Hereafter, details of the embodiments according to the presentinvention will be explained with reference to the drawings.

[0027]FIG. 1 shows the schematic circuit composition of a speed-controlapparatus for an induction motor representing an embodiment according tothe present invention. The apparatus includes an induction motor 1, apower-conversion unit 2 for outputting output voltages proportional tovoltage instruction values V1*, and a coordinate-transformation unit 3for transforming the coordinates of output currents iu and iw andcalculating d-axis and q-axis currents Id and Iq. A speed-estimationunit 4 is provided for calculating an estimated speed ωr{circumflex over( )} based on a q-axis voltage instruction value ωr{circumflex over ( )}and the current Iq. A speed-control unit 5 is provided for outputting aq-axis current instruction value Iq* corresponding to a differencebetween a speed instruction value Vq** and the estimated speedωr{circumflex over ( )}, which further includes a limiter for limitingthe instruction value Iq* corresponding to the value of Id. A q-axiscurrent-control unit 6 is provided *for outputting Δq corresponding tothe values Iq* and Iq, and a slip frequency-calculation unit 7 isprovided, in which a slip frequency calculator 71 is included forobtaining a calculated slip frequency based on the value Iq*. Aswitching unit 8 is provided, which includes a multiplier 82 formultiplying ωr{circumflex over ( )} by the output Gal of a functiongenerator 81, a multiplier 84 for multiplying ωr{circumflex over ( )} bythe output Ga2 of a function generator 83, and an adder 85 for summingthe outputs from both the multipliers 81 and 84, for switching itsoutput ωr{circumflex over ( )}{circumflex over ( )} betweenωr{circumflex over ( )} and ωr* corresponding to the value of the speed.An adder 9 is provided for obtaining a signal ω1{circumflex over ( )} bysumming the output signal ωr{circumflex over ( )}{circumflex over ( )}of the switching unit 8 and ωs*, and a phase-generation unit 10 isprovided for outputting a phase reference value θ by integrating theoutput frequency instruction value ω1* of output from the adder 9. Ad-axis current instruction unit 11 is provided, including a multiplier112 for multiplying an additional current value ΔId by the output Ga3 ofa function generator 111 and an adder 113 for obtaining a d-axis currentinstruction value Id** by summing a reference current value Id* and theoutput of the multiplier 111. A d-axis current-control unit 12 isprovided for outputting a signal Δd corresponding to a differencebetween Id** and Id and a voltage-calculation unit 13 is provided forcalculating d-axis and q-axis reference voltages Vd* and Vq* based onId**, Iq*, and ω1*. Further, adders 14 and 15 are provided foroutputting Vd** by summing Vd* and Δd and a coordinate-transformationunit 16 is provided for outputting output-voltage instruction values V1*(for three phases) by transforming the coordinates of Vd** and Vq**.

[0028] In the above units, the units 8 and 11 are specific to thisembodiment. The performances of the function generators included in therespective units 8 and 11 are as follows. The output Ga1 of the functiongenerator 81 is 0 near the input value 0, and 1 in the range of a largeinput value, and vice versa as to the output Ga2 of the functiongenerator 83. The outputs Ga1 and Ga2 are complementary to each other,which is expressed by the equation (1).

Ga1+Ga2=1  (1)

[0029] Therefore, the output Gr of the switching unit 8 is given by theequation (2).

ωr{circumflex over ( )}{circumflex over ( )}=ωr{circumflex over( )}·Ga1+ωr*·Ga2  (2)

[0030] Further, the output Ga3 of the function generator 111 is 0 if ωr*is near 0, and 1 if ωr* is in the range of a large value. Accordingly,Id** and Id are increased from the reference value Id* by ΔId. Thegradual increase and decrease regions of Ga3 are prepared to smoothlychange Id**, and an intermediate value between Id* and Id*+ΔId is outputas Id**.

[0031] The operation of the induction motor control system according tothis embodiment will be explained below. The operations of the units ordevices 1-7, 9, 10, and 12-16 are the same as those in the conventionalvector control system without a speed sensor. First, the outline of theconventional vector control system without a speed sensor will beexplained.

[0032] In the conventional vector control system without a speed sensor,the speed is estimated based on the output voltage and current of thepower-conversion unit 2, and the speed is controlled by feeding-back theestimated speed ωr{circumflex over ( )} to the speed-control unit 5.Further, the output frequency of the power-conversion unit 2 iscontrolled based on the sum of the estimated speed ωr{circumflex over( )} and the calculated slip frequency ωs*. The difference between thevector control without a speed sensor and the well-known vector controlwith a speed sensor is that, in the vector control system without aspeed sensor, the estimated speed is used in place of the speed detectedby a speed sensor mounted on the induction motor 1. However, thefundamental operation is common to both the controls.

[0033] To control the current flows Id and Iq in the induction motor 1according to the d-axis current instruction value and the q-axis currentinstruction value output from the speed-control unit 5, it is necessaryto feed the required voltage to the induction motor 1 from thepower-conversion unit 2. Therefore, the voltage-calculation unit 13calculates the d and q-axis voltage reference values Vd* and Vq* basedon the current instruction values Id** and Iq*, and the output frequencyinstruction value ω1*, and the output voltage of the power-conversionunit 2 is controlled according to the calculated voltage referencevalues Vd* and Vq*. However, since the current flows Id and Iq agreewith their instruction values due to control errors by performing onlythe above control, the reference voltage values Vd* and Vq* arecorrected with the Δd and Δq output from the d and q-axiscurrent-control units 12 and 6 so that the current flows Id and Iq agreewith their instruction values. In this way, the slipfrequency-control-type vector control is performed, and the torque ofthe induction motor 1 is controlled in proportion to Iq*.

[0034] In the following, the detailed operation of each unit or devicewill be explained.

[0035] The speed-estimation unit 4 calculates the estimated speed valueωr{circumflex over ( )} based on the equation (3).

ωr{circumflex over ( )}={1/(1+TO·s)}L2*/(M*−φ2d)}{Vq**−ω1*·Lσ*·Id**−(Rσ*+Lσ*·s)Iq}  (3)

[0036] where, T0: a time constant of the observer; L2*, M*: secondaryand exciting inductance values (reference values); φ2d*: a secondaryq-axis flux (reference value); Rσ*: a sum of primary and secondaryresistance values (reference value); Lσ*: a sum of primary and secondaryleakage inductance values (reference value); and ω1*: the outputfrequency of the power- transformation unit 2 (instruction value).

[0037]FIG. 2 shows the calculational function performed based on theequation (3) by the speed estimator 4. Reference number 41 indicates themodel of the inductance motor 1, which shows the relationship among theq-axis current Vq (=Vq**) of the motor 1, the induced-electromotiveforce Eq, and the current Iq. In the method of estimating ωr{circumflexover ( )}, Eq is estimated with the inverse model of the motor 1, andωr{circumflex over (1)} obtained by dividing the estimated Eq by thereference flux.

[0038] The estimated value ωr{circumflex over ( )} is used as a feedback signal to the speed-control unit 5, and for the calculation of ω1*.The equation (4) used for the calculation of ω1* is shown below. In theconventional control method, ωr{circumflex over ( )} is directly used asthe output frequency reference value ω1 to control the output frequencyof the power-conversion unit 2.

ω1*=ωr{circumflex over ( )}+ωs*  (4)

[0039] In the speed-control unit 5, the q-axis current instruction valueIq* is calculated corresponding to the speed deviation (ωr*−ωr*). Sincethe torque of the motor 1 is basically proportional to Iq*, the speed iscontrolled such that ωr* agrees with ωr{circumflex over ( )}. In orderthat the torque of the motor 1 is precisely proportional to Iq*, it isrequired that the current value Iq of the motor 1 agrees with Iq*, andthe flux in the motor 1 is kept at the reference value. To attain theabove conditions, it is necessary to control the current values Id andIq of the motor 1 so as to agree with the respective instruction valuesId** and Iq*. To implement the above control, the d and q-axiscurrent-control units 12 and 6 are equipped. Although the voltage valuesVd and Vq of the motor 1 under various operational conditions areexpressed by the equations (5), the reference voltage values Vd* and Vq*corresponding to Vd and Vq can be calculated in advance with theequations (6) using Id**, Iq*, ω1*, and the characteristic parameters ofthe motor 1. This calculation is performed by the voltage-calculationunit 13.

Vd=r1·Id−ω1·Lσ·Iq

Vq=r1·Iq+ω1·Lσ·Id+ω1(M/L2)φ2d  (5),

[0040] where r1: a primary resistance value (actual value); Lσ: a sum ofprimary and secondary leakage inductance values (actual value); L2, M:secondary and exciting inductance values (actual values); and φ2d: asecondary q-axis flux (reference value).

Vd′=r1*·Id**−ω1*·Lσ*·Iq*

Vq*=r1*·Iq*+ω1*·Lσ*·Id**+ω1*(M*/L2*)φ2d*  (6)

[0041] where * and ** indicate a reference value and an instructionvalue, respectively.

[0042] The output voltage of the power-conversion unit 2 (the voltage inthe motor 1) is basically controlled according to Vd* and Vq*. If acontrol error exists, the actual current values Id and Iq do not agreewith the respective instruction values by performing only the abovecontrol. Therefore, the adjustment signals Δd and Δq corresponding tothe respective current deviations are obtained by the d and q-axiscurrent-control units 12 and 6, and the output voltage of thepower-conversion unit 2 is corrected based on the adjustment signals Δdand Δq so that Id and Iq agree with the respective instruction values.The operations which are explained up to here are common to theconventional control method.

[0043]FIG. 3 shows the relationship between the speed ωr and the torqueτm generated in an induction motor controlled by a conventional controlmethod in the speed range near zero. The shadowed region in this figureindicates an unstable region in which the decrease of the torque easilyhappens.

[0044] In the shadowed region, the range 0.5-1 Hz of speed forcorresponds to an unstable area in the motoring region in which thesigns of τm and ωr are the same, and the range less than several Hz ofspeed cor corresponds to an unstable area in the regeneration region inwhich the sign of τm is different from the sign of ωr. Also, if theestimation error in ωr{circumflex over ( )} obtained by thespeed-estimation unit 4 increases, the shadowed region is extended,which in turn sometimes makes the on-load operation of the motor 1 at alow speed impossible.

[0045] The estimation error in ωr{circumflex over ( )} is caused bychanges in the temperature of the primary and secondary resistancevalues; changes in the leakage-inductance values due to the fluxsaturation in the iron core of the motor 1; etc. Specifically, thetorque of the motor 1 easily decreases in the speed range near zero dueto various kinds of causes.

[0046] The control method of this embodiment, which is aimed atpreventing the decrease of the torque, controls the motor 1 in the speedrange near zero with a control principle different from the aboveconventional control method. This control principle is mentioned below.

[0047] The decrease of the torque is mainly caused by the error in theestimated speed, and two main causes bring about this error.

[0048] (1) since the frequency is controlled based on the estimatedspeed, the actual slip frequency is deviated from the proper value dueto the estimation error.

[0049] (2) Since the speed is controlled based on the estimated speed,the estimation error makes it impossible to control the torque currentat the proper value.

[0050] This embodiment seeks to solve the above problems in accordancewith the following control strategies.

[0051] Strategy 1: the output frequency instruction value ω1* iscalculated with the speed instruction value ωr* in place of theestimated speed ωr{circumflex over ( )}.

[0052] That is, the output frequency of the power-conversion unit 2 iscontrolled according to the speed instruction value ωr* in the speedregion near zero by outputting ωr* from the switching unit 8 in placeωr{circumflex over ( )} used in the ordinary speed region.

[0053] Strategy 2: the output current of the power-conversion unit 2 iscontrolled at a predetermined value larger than that in the ordinaryno-load operation.

[0054] For example, the q-axis current is set to zero, and the d-axiscurrent is controlled at a predetermined value larger than that in theordinary no-load operation. In this control, ΔId is added to thereference value by the d-axis current instruction unit 11 so as tocontrol the current Id at a larger value.

[0055] In the case when the strategies 1 and 2 are adopted, therelationship between the torque generated by the motor 1 and the currentI1 is shown in the equation (7).

τm=K(ωs·T2)/(1+(ωs·T2)²)I1²  (7),

[0056] where K: a proportional constant; ωs: the slip frequency; T2: asecondary time constant; and I1: the primary current in the motor 1.

[0057] If I1 is constant, the torque Tm generated by the motor 1 ismaximum when ωr·T2=±1, and τm changes corresponding to the value of ωsbetween 0 and +1. The slip frequency ωr changes corresponding to changesin the actual speed ωr in response to the output frequency ω1 (=(ωr*) .That is, since ωr increases or decreases corresponding to the increaseor decrease of a load torque, the torque is generated following a loadtorque. Consequently, the speed ωr of the motor 1 is kept near ωr*(deviates by a slip). Thus, the speed is controlled according to thespeed instruction value.

[0058] Here, since the maximum torque of the motor 1 is required to belarger than the maximum load-torque, it is necessary to control I1 so asto be a value larger than that corresponding to the maximum load-torque.Therefore, Id or Iq is controlled to attain such a value.

[0059] Although Id or Iq can be set to the predetermined valueindependent of the speed deviation, since detecting the direction of theload torque based on the estimated speed ωr{circumflex over ( )} isdifficult because of the bad estimation accuracy of speed ωr{circumflexover ( )} in the speed range near zero, the polarity of Iq* cannot beset. Therefore, a method of setting Id** to the predetermined value, forwhich setting of the polarity is not necessary, is used in theembodiment shown in FIG. 1. In this method, as mentioned in Strategy 2,Iq* is set to 0, and Id** is set to the sum of the reference value Id*in the ordinary speed region and ΔId to control Id (corresponds to I1)such that Id corresponds to the maximum load-torque.

[0060] In the speed range near zero, since the output frequency andcurrent of the power-conversion unit 2 are controlled as mentionedabove, the above problems (1) and (2) are solved, which in turn solvesthe problem of the shortage in torque.

[0061]FIG. 4 shows the relationship between the speed and the torquegenerated by the induction motor 1 in this embodiment. The unstableregion (shadowed region) shown in FIG. 3 disappears in FIG. 4. Althoughthe speed changes by a slip frequency, a high torque value can beachieved even in the speed region including and near zero.

[0062] In the regions corresponding to frequency values less thannegative several Hz and more than positive several Hz, the output or theswitching unit 8 shown in FIG. 1 is switched from ωr* to ωr{circumflexover ( )} and the frequency is controlled using the estimated speed bythe same method as that in the conventional control. Moreover, tosmoothly switch the output of the switching unit 8, the switchingbetween ωr* and ωr{circumflex over ( )} is gradually performed so as toprevent a rapid change due to the switching ω1*. The gradual increaseand decrease characteristics for the outputs Ga1 and Ga2 of the functiongenerators 81 and 82 are prepared to implement the above objective.Also, in the d-axis current-instruction unit, The gradual increase anddecrease characteristics for the output Ga3 are prepared to prevent arapid change in Id*. Further, in the operational state in which Idshould be enhanced (the speed region near zero), since it is necessaryto restrict Id* such that the current I1 in the motor 1 does not exceedthe rated value, and Iq* deviates from the proper value due to thedegradation in accuracy in the estimated speed ωr{circumflex over ( )},Iq* is required to be set to a predetermined value or almost zero. Inthis embodiment, the limit value IqMax of Iq* is changed correspondingto Id based on the equation (8).

IqMAX={square root}{square root over (I1*²−Id²))}  (8)

[0063] where I1* is the setting value of current in the motor 1.

[0064]FIG. 5 shows a schematic block diagram of the circuit compositionof a speed-control apparatus for the induction motor 1 representinganother embodiment according to the present invention. This embodimentis an application example of a vector control apparatus without a speedsensor, in which the estimated speed ωr{circumflex over ( )} is obtainedfrom the output of the q-axis current-control unit 6 a. In this figure,the units or devices 1 -3, 5, 7-14, and 16 are the same as those inFIG. 1. Reference number 6 a indicates a q-axis current-control unit foroutputting ωr{circumflex over ( )} corresponding to the deviationbetween Iq* and Iq, and the switching unit 8 selects and outputs one ofωr* and ωr{circumflex over ( )} depending on the value of ωr* as well asthat in the previous embodiment. In the ordinary speed range, as well asin the conventional control, since ωr{circumflex over ( )} is outputfrom the switching unit 8, and the output of the current-control unit 6a also corresponds to ωr{circumflex over ( )}, it is evident that thisembodiment functions in the same manner as the previous embodiment, andthe same effects can be obtained.

[0065] Further, FIG. 6 shows a schematic block diagram of the circuitcomposition of a speed-control apparatus for the induction motor 1 ofanother embodiment according to the present invention. This embodimentis an application example of a vector control apparatus without a speedsensor, in which the estimated speed ω1* is obtained from the output ofthe q-axis current-control unit 6 b. In this figure, the units ordevices 1-3, 5, 7-14, and 16 are the same as those in FIG. 1. Referencenumber 6 a indicates a q-axis current-control unit for outputting ω1*corresponding to the deviation between Iq* and Iq, and reference number9 a indicates a subtracter for obtaining the estimated speedωr{circumflex over ( )} by subtracting ωr* from ω1* and for feeding backωr{circumflex over ( )} to the speed-control unit 5. The switching unit8 selects and outputs one of ωr* and ωr{circumflex over ( )} dependingon the value of ωr* as well as that in the previous embodiment. In theordinary speed range, as well as in the conventional control, sinceωr{circumflex over ( )} is output from the switching unit 8, and theoutput of the current-control unit 6 b also corresponds to ω1*, it isevident that this embodiment functions in the same manner as the aboveembodiments, and the same effects can be obtained.

[0066] Although Id is controlled to attain the predetermined enhancedvalue in the speed range near zero, if both the positive and negativedirections of the torque possibly exist in the speed range near zero,and the direction is not fixed, the control method of this embodiment,in which Iq* is set to zero, and Id is enhanced, is suitable for thiscase. On the other hand, if there is only one direction of the torque,since the polarity of Iq* can be set corresponding to the direction ofthe torque, it is possible to set Iq* to a predetermined value(corresponds to the maximum load-torque) in place of setting Id to apredetermined value as performed in the previous embodiment in the speedrange near zero.

[0067] Furthermore, FIG. 7 shows a schematic block diagram of thecircuit composition of a speed-control apparatus for the induction motor1 representing another embodiment according to the present invention. Inthis figure, the compositions and operations of the units or devices1-10, and 12-16 are the same as those in FIG. 1. Reference number 17indicates a q-axis current instruction unit for outputting the sum ofthe set current value Iq0 which is modified depending on the value ofωr* and the output Iq* of the speed-control unit 5, and this q-axiscurrent instruction unit 17 includes a multiplier 172 for multiplyingIq0 by the output Ga4 (0≦Ga4≦1) of a function generator 171 with thegradual increase and decrease characteristics, and an adder 173 foroutputting Iq** by adding Iq* to the output of the multiplier 172.

[0068] The operation of the q-axis current instruction unit 17 will beexplained below. Since Ga4 is “1” in the speed region near zero, andpossesses the gradual increase and decrease characteristics (0≦Ga4≦1) inthe region other than the speed region near zero, Iq0 is output from theq-axis current instruction unit 17 in the speed region near zero.Therefore, Iq is controlled according to Iq0, and a sufficient torquecan be obtained (Iq0 is set to a value which corresponds to the maximumload-torque.) On the other hand, in the region other than the speedregion near zero, Iq is controlled based on Iq, which is the same as theconventional control.

[0069] In this way, in the speed range near zero, since the outputfrequency of the power-conversion unit 2 is controlled according to ωr*,and the current in the motor 1 is controlled based on the predeterminedvalue Iq0, the same effects of the above embodiments can be obtainedwith this embodiment.

[0070] In the above embodiments, although the speed-control unit 5 isprovided, and the control methods in those embodiments are applied to aspeed-control method in which the torque is controlled based on Iq* orIq** output from the speed-control unit 5, the present invention can beapplied to a control method which operates without the speed-controlunit 5.

[0071]FIG. 8 shows a schematic block diagram of the circuit compositionof a speed-control apparatus for the induction motor 1 representinganother embodiment without the speed-control unit 5. In this figure, theunits or devices 1-3, 10-14, and 16 are the same as those in FIG. 1.Reference number 7 a 1 indicates a slip frequency calculator forobtaining the calculated slip frequency ωs* based on the q-axis currentvalue Iq, and reference number 9 b indicates an adder for obtaining thesignal ω1* by adding the speed instruction value ωr* to the signal ωr*.

[0072] The operation of the control system shown in FIG. 8 will beexplained below. In the region other than the speed region near zero,the frequency instruction value ω1* (=ωr*+ωs*) is output, and the d-axiscurrent instruction value Id** is output from the d-axis currentinstruction unit 11. Thus, the operation of this system is the same asthat of the conventional vector control without a sensor. That is, theoutput frequency of the power-conversion unit 2 is controlled mostlyaccording to ωr*, and the output voltage of the power-conversion unit 2is also controlled based on the necessary voltage of the motor 1 whichis calculated based on Id**, Iq, and ω1* by the voltage-calculation unit13.

[0073] Since the output voltage and frequency of the power-conversionunit 2 are controlled as explained above, an operation similar to thatof a V/f control is performed.

[0074] However, since the induced-electromotive-force (the magnetic fluxin the motor 1) is controlled so as to attain a predetermined value bycompensating the internal voltage decrease in the motor 1 with thevoltage-calculation unit 13 in this embodiment, a sufficient quantity oftorque can be obtained to the speed range near zero.

[0075] In the speed range near zero, the d-axis current instruction unit11 outputs the instruction value Id** obtained by adding ΔId to Id* inorder to enhance Id. By this control, in this embodiment as well as theprevious embodiment, the frequency instruction value ω1* is controlledaccording to the speed reference value ωr*, and the d-axis current iscontrolled so as to have a higher value than that in the ordinary speedrange. Accordingly, the shortage of the torque can be eliminated.

[0076] In the above embodiments, since Iq* is controlled to be zero inthe speed range near zero, ωs* is zero. Therefore, the output frequencyw1 agrees with the speed instruction value ωr*. Thus, if a load torqueis applied to the motor 1, the speed ωr of the motor 1 deviates from ωr*by the slip frequency ωs. This deviation can be compensated byestimating the slip frequency using the voltage instruction values Vd**and Vq** in the embodiment shown in FIG. 1, or the outputs Δd and Δq ofthe current-control units 12 and 6, which are obtained based on theoutput voltage values of the power-conversion unit 2, and by adding theestimated slip frequency ωr{circumflex over ( )} to the frequencyinstruction value.

[0077] Further, the estimated slip frequency obtained based on theoutput voltage values is added to the calculated slip frequency obtainedusing the current instruction value Iq. Furthermore, this sum is used asa new calculated slip frequency to correct the frequency instructionvalue, and this makes it possible to compensate the deviation of thespeed due to the load torque in the whole speed range from zero.

[0078]FIG. 9 shows a schematic block diagram of the circuit compositionof a speed control apparatus for the induction motor 1 representinganother embodiment which implements the above-mentioned control. Thisembodiment applies the above control to the vector control apparatuswithout a speed sensor shown in FIG. 1.

[0079] In this figure, the units or devices 1-6, and 8-16 are the sameas those in FIG. 1. Reference number 7 b indicates a slipfrequency-calculation unit for calculating the slip frequency ωs* andthe estimated slip frequency ωs{circumflex over ( )} and obtaining thesum ωs** of ωs* and ωs{circumflex over ( )}. Further, there is provideda slip frequency calculator 7 b 1 for obtaining ωs{circumflex over ( )}*with the current instruction value Iq*; a slip frequency estimator 7 b 2for obtaining ωs{circumflex over ( )} with the outputs Δd and Δq of thecurrent-control units 12 and 6, and the output frequency instructionvalue ω1*; and an adder 7 b 3 for obtaining the sum of ωs* andωs{circumflex over ( )}.

[0080] Here, the output of the adder 9 agrees with (ωr*+ωs{circumflexover ( )}) in the speed range near zero, and with (ωs{circumflex over( )}·Ga1+ωr*·Ga2+ωs{circumflex over ( )})+ωs*) in the range other thanthe speed range near zero.

[0081] The slip frequency estimator 7 b 2 will be explained below.

[0082] First, the composition of the estimator 7 b 2 will be explainedwith reference to FIG. 10.

[0083] The ω1* input to the estimator 7 b 2 is multiplied by acoefficient (M*/L2*), and further by a d-axis flux reference value φ2d*,and the multiplication result is input to an adder 7 b 22 along with Δq.Furthermore, Δq and the output signal of the adder 7 b 22 are input to adivider 7 b 23. The output signal of the divider 7 b 23 is multiplied bythe reciprocal (1/T2*) of the secondary time constant of the motor 1,and the estimated slip frequency ωs{circumflex over ( )} is output fromthe slip frequency estimator 7 b 2.

[0084] Next, the effects of this slip frequency estimator 7 b 2 will beexplained.

[0085] The voltage instruction values Vd** and Vq**, and the voltagevalues Vd and Vq in the motor 1 are expressed by the equations (9) and(10).

Vd**=r1*·Id**−ω1*·Lσ*·Iq*+Δd

Vq**=r1*·Iq*+ω1*·Lσ*·Id**+ω1*(M*/L2*)φ2d*+Δq  (9)

Vd=r1·Id−ω1·Lσ·Iq−ω1(M/L2)φ2q

Vq=r1·Iq+ω1·Lσ·Id+ω1(M/L2)φ2d  (10)

[0086] Here, since the equations (9)=the equations (10), the outputs ofthe current-control units 12 and 6 are expressed by the equations (11).

Δd=(r1−r1*)Id−ω1(Lσ−Lσ*)Iq−ω1(M/L2)φ2q

Δq=(r1−r1*)Iq+ω1(Lσ−Lσ*)Id+ω1((M/L2)φ2d−(M*/L2*)φ2d*)  (11),

[0087] provided that ω1*=ω1, Id**=Id, and Iq*=Iq.

[0088] Since the q-axis current Iq is controlled to be zero in the speedrange near zero, if Iq=0, or Lσ≈Lσ*, the second term is sufficientlysmall to be neglected in comparison with the third term in the equations(11).

[0089] Then, Δd and Δq in the equations (11) are expressed by theequations (12).

Δd≈(r1−r1*)Id−(ω1(M/L2)φ2q

Δg≈ω1{(M/L2)φ2d−(M*/L2*)φ2d*}  (12)

[0090] Thus, Δd almost agrees with the induced-electromotive force Ed(=ω1(M/L2)φ2q) related to the q-axis flux φ2q.

[0091] On the other hand, if the induced-electromotive force referencevalue ω1(M*/L2*)φ2d* is added to Δq, the induced-electromotive force Ed(=ω1((M/L2)φ2d) related to the d-axis flux φ2d is obtained.

[0092] From the equations (12), the equation (13) is obtained.

Δq+ω1*(M*/L2*)φ2d*=ω1(M/L2)φ2d  (13)

[0093] Here, if Id and Iq are controlled such that Id is a predeterminedvalue, and Iq=0, in the above method, the relationship among the fluxesφ2d and φ2q, and the slip frequency ωs in the motor 1 is expressed bythe equation (14). $\begin{matrix}\begin{matrix}{{\omega \quad s} = \quad {\left( {1/{T2}} \right)\quad \left( {{- \phi}\quad 2\quad {q/\phi}\quad 2\quad q} \right)}} \\{= \quad {\left( {1/{T2}} \right)\quad \left( {{ed}/{eq}} \right)}}\end{matrix} & (14)\end{matrix}$

[0094] Further, by performing the calculation indicated by the equation(15), the estimated slip frequency ωs{circumflex over ( )} can beobtained.

ωs{circumflex over ( )}=(1/T2*){Δd/(Δq+ω1*(M*/L2*)φ2d*)}  (15)

[0095] On the other hand, in the range other than the speed range nearzero, the current instruction value Iq* is generated by thespeed-control unit 5. In this region, the estimated slip frequencyωs{circumflex over ( )} is obtained using Iq* as shown by the equation(16).

ωs{circumflex over ( )}=Iq*·M/(T2*·φ2d)  (16)

[0096] Here, the output frequency instruction value ω1* is obtained byadding the sum ωs** of ωs{circumflex over ( )} obtained by the equation(15) and ωs* obtained by the equation (16) to the output ωs{circumflexover ( )}{circumflex over ( )} of the switching unit 8.

ω1*=ωr{circumflex over ( )}{circumflex over ( )}+ωs*+ωs{circumflex over( )}  (17)

[0097]FIG. 11 shows the relationship between the speed and the torquegenerated in an induction motor controlled using the above slipfrequency-compensation method according to the present invention. Bymeans of this slip frequency-compensation method, the deviation of thespeed can be corrected corresponding to the torque from the speed rangenear zero (ωr≈0), which in turn makes it possible to achieve a highlyaccurate speed control of the motor 1.

[0098] Moreover, although ωs{circumflex over ( )} is calculated using Δdand Δq in this embodiment shown in FIG. 9, ωs{circumflex over ( )} canalso be obtained using the induced-electromotive force valuesed{circumflex over ( )} and eq{circumflex over ( )} calculated from thevoltage instruction values Vd** and Vq**.

[0099] By subtracting the resistance voltage-decrease r1·Id and theleakage inductance voltage-decrease ω1·Lσ·Id from Vd and Vq,respectively, the respective values ed{circumflex over ( )} andeq{circumflex over ( )} can be obtained as shown by the equations (18).ed{circumflex over ( )}. Here, in the speed range near zero, Iq=0.$\begin{matrix}\begin{matrix}{{ed}^{\quad\hat{}} = \quad {{Vq}^{**} - {{r1}^{*} \cdot {Id}^{*}} + {{- \omega}\quad 1^{*}\left( {m^{*}/{L2}^{*}} \right)\phi \quad 2{q\quad}^{\hat{}}}}} \\{{ed}^{\quad\hat{}} = \quad {{Vd}^{**} - {{{\omega 1}^{*} \cdot L}\quad {\sigma^{*} \cdot {Id}^{**}}}}} \\{= \quad {{{\omega 1}^{*}\left( {M^{*}/{L2}^{*}} \right)}\phi \quad 2{d\quad}^{\hat{}}}}\end{matrix} & (18)\end{matrix}$

[0100] Since as is obtained by the equation (14), ωs{circumflex over( )} can be calculated by the equation (19) using φ2d{circumflex over( )} and φ2q{circumflex over ( )}, or ed{circumflex over ( )} anded{circumflex over ( )}. $\begin{matrix}\begin{matrix}{{\omega \quad s^{\hat{}}} = \quad {\left( {1/{T2}^{*}} \right)\quad \left( {{- \phi}\quad 2\quad {q^{\hat{}}/\phi}\quad 2\quad q^{\hat{}}} \right)}} \\{= \quad {\left( {1/{T2}^{*}} \right)\quad \left( {{ed}^{\quad\hat{}}/{eq}^{\hat{}}} \right)}}\end{matrix} & (19)\end{matrix}$

[0101] By controlling the speed according to ω1 calculated by theequation (17) using the obtained ωs{circumflex over ( )} and ωs*, thesame effects as those of the speed control performed based on theequation (15) can be achieved.

[0102] Although this control method is applied to the control apparatusshown in FIG. 1 in the above embodiment shown in FIG. 9, if thisinvention is applied to the control apparatuses shown in FIGS. 5, 6, and8, ωs{circumflex over ( )} is calculated p based on the voltageinstruction value Vq* and the output Δd of the current-control unit 12.

[0103] That is, since Vq*=Vq (the second one of the equations (6) thesecond one of the equations (10)), Iq=0, and the effect of leakageinductance voltage-decrease is small enough to be neglected (even ifLσ≈Lσ*), Vq* is expressed by the equation (20).

Vq*≈ω1(M/L2)φ2d  (20)

[0104] Further, since Δd is obtained by the equations (12),ωs{circumflex over ( )} can be calculated by the equation (21) using Δdand Vq*.

ωs{circumflex over ( )}=(1/T2*)(Δd/Vq*)  (21)

[0105] By controlling the speed according to ω1 calculated by theequation (17) using the obtained ωs{circumflex over ( )} and ωs*, thesame operation and effects as those of the embodiment shown in FIG. 9which is controlled based on the equation (15) can be achieved.

[0106] Although the slip frequency-calculation unit 7 b shown in FIG. 9is used in the above embodiments, the same effects as those of the aboveembodiment can be obtained using a slip frequency-calculation unit 7 cshown in FIG. 12.

[0107] The unit 7 c is the slip frequency-calculation unit forcalculating the sum ωs** of ωs* and ωs{circumflex over ( )}, whichincludes a slip frequency calculator 7 c 1 for obtaining ωs* using Idand φ2d{circumflex over ( )}; a slip frequency estimator 7 c 2 forobtaining ωs{circumflex over ( )} using φ2d and φ2q{circumflex over( )}; an adder 7 c 3 for adding φs{circumflex over ( )} to ωs*; a d-axisflux estimator 7 c 4 for calculating φ2d{circumflex over ( )} using Δqand ω1*; and a q-axis flux estimator 7 c 5 for calculatingφ2q{circumflex over ( )} using Δd and ω1*.

[0108] First, the d-axis flux estimator 7 c 4, which is one of theelements composing the slip frequency-calculation unit 7 c, will beexplained below with reference to FIG. 13.

[0109] The ω1* input to the estimator 7 c 4 is multiplied by thecoefficient (M*/L2*), and further by the d-axis flux reference valueφ2d*, and the multiplication result is input to the adder 7 c 42 alongwith Δq. Further, the output of the adder 7 c 42 and the value ω1*(M*/L2*) are input to a divider 7 c 43, and the divider 7 c 43 outputsthe estimated flux φ2d.

[0110] Next, the q-axis flux estimator 7 c 5 will be explained belowwith reference to FIG. 14.

[0111] The ω1* input to the estimator 7 c 5 is multiplied by thecoefficient (M*/L2*), and the multiplication result is input to thedivider 7 c 52 along with Δd. Further, the divider 7 c 52 outputs theestimated flux φ2q{circumflex over ( )}.

[0112] The composition of the slip frequency estimator 7 c 2 forcalculating ωs{circumflex over ( )} using the calculated φ2d and φ2q isshown in FIG. 15.

[0113] The estimated values φ2d{circumflex over ( )} and φ2q{circumflexover ( )} input to the slip frequency estimator 7 c 2 are input to adivider 7 c 21.

[0114] The output signal of the divider 7 c 21 is multiplied by thereciprocal (I/T2*) of the secondary time constant of the motor 1, andthe estimator 7 c 2 outputs the estimated slip frequency ωs{circumflexover ( )}. Further, the frequency instruction value is corrected withthe obtained ωs{circumflex over ( )} and ωs*.

[0115] In this control method, the estimated fluxes φ2d and φ2q areobtained by the equations (22) based on the output voltage values, andωs{circumflex over ( )} and ωs* are further calculated using theestimated fluxes φ2d{circumflex over ( )} and φ2q{circumflex over ( )}as shown by the equations (23). Thus, the speed is controlled accordingto ω1* which is calculated based on the equations (17) using theobtained values ωs{circumflex over ( )} and ωs*.

φ2d=[{Δq+ω1*(M*/L2*)φ2d*}/(ω1*M*/L2*)]

φ2q={Δq/(ω1*M*/L2*)}  (22)

ωs=(1/T2*)(−φ2q/φ2q{circumflex over ( )})

ωs*=Iq*·M*/(T2*·φ2d{circumflex over ( )})  (23)ps

[0116] In this control method also, the same operation and effects asthose of the embodiment shown in FIG. 9 which is controlled based on theequation (14) can be achieved.

[0117] Further, by using a slip frequency-calculation unit 7d shown inFIG. 16 in place of the slip frequency-calculation unit 7 b shown inFIG. 9, the same effects can also be obtained.

[0118] The unit 7 d is a slip frequency-calculation unit for obtainingthe sum ωs** of ωs* and ωs{circumflex over ( )}, which includes a slipfrequency calculator 7 d 1 for obtaining ωs* using Iq* andφ2d{circumflex over ( )}; a slip frequency estimator 7 d 2 for obtaining(s using φ2d{circumflex over ( )} and φ2q{circumflex over ( )}; an adder7 d 3 for adding ωs{circumflex over ( )} to ωs*; and a flux estimator 7d 4 for calculating φ2d{circumflex over ( )} and φ2q{circumflex over( )} using Vd** and Vq**, and ω1*.

[0119] The composition of the flux estimator 7 d 4, which is one ofelements composing the unit 7 d, is shown in FIG. 17.

[0120] Vd**, the calculated resistance voltage-decrease value (r1*·Id*),and the calculated leakage inductance voltage-decrease value(−ω1*·Lσ*·Iq*) are input to a subtracter 7 d 41. Further, by multiplyingω1* by the coefficient (M*/L2*), and further by φ2q, the estimatedd-axis induced-electromotive force ed{circumflex over ( )} is obtained.The obtained value ed{circumflex over ( )} is input to a subtracter 7 d43 along with the output signal of the subtracter 7 d 41. Furthermore,the output signal of the subtracter 7 d 43 is input to an integrator 7 d44, and the estimated flux φ2d{circumflex over ( )} is output from theintegrator 7 d 44.

[0121] Also, Vq**, the calculated resistance voltage-decrease value(r1*·Iq*), and the calculated leakage inductance voltage-decrease value(−ω1*·Lσ*·Id**) are input to a subtracter 7 d 45.

[0122] Further, by multiplying ω1* by the coefficient (M*/L2*), andfurther by ω2d{circumflex over ( )}, the estimated q-axisinduced-electromotive force eq{circumflex over ( )} is obtained. Theobtained value eq{circumflex over ( )} is input to a subtracter 7 d 46along with the output signal of the subtracter 7 d 45. Furthermore, theoutput signal of the subtracter 7 d 46 is input to an integrator 7 d 47,and the estimated flux φ2q is output from the integrator 7 d 47.

[0123] In this speed control method, φ2d and φ2q are obtained by theequations (24) based on the output voltage, and ωs{circumflex over ( )}is then calculated.

φ2d{circumflex over( )}=∫[Vd**−r1·Id**+ω1*·Lσ*·Iq*−ω1*(M*/L2*)φ2q{circumflex over ( )}]dt

φ2q{circumflex over( )}=≠[Vq**−r1·Iq*+ω1*·Lσ*·Id*+ω1*(M*/L2*)φ2d{circumflex over( )}]dt  (24)

[0124] That is, φ2d{circumflex over ( )} and φ2q{circumflex over ( )}are obtained with the flux estimator 7 d 4 using the voltage instructionvalues Vd** and, and Vq**, and the calculator 7 d 1 and the estimator 7d 2 shown in FIG. 16 calculate ωs* and ωs{circumflex over ( )},respectively, according to the equations (23). Further, the adder 7 d 3adds ωs* to ωs{circumflex over ( )}, and the frequency instruction valueω1* is corrected with the sum

[0125] In this control method also, the same operation and effects asthose of the embodiment shown in FIG. 9 can be achieved.

[0126] Meanwhile, the respective calculational functions of thecalculator 7 d 1 and the estimator 7 d 2 are the same as those of thecalculator 7 c 1 and the estimator 7 c 2.

[0127] In the above embodiments, the d-axis current Id is controlled tobe constant independent of the load torque, but the operationalefficiency deteriorates in an operation with light load-torque. Here,the operational efficiency in the operation with light load-torque canbe improved by correcting the current instruction value Id**,corresponding to an estimated torque τm{circumflex over ( )}.

[0128]FIG. 18 shows the circuit composition of a speed-control apparatusfor an induction motor of an embodiment in which this speed controlmethod is used. In this embodiment, the above q-axis current-correctionmethod according to this control method is applied to the controlapparatus shown in FIG. 1.

[0129] In this figure, the units or devices 1-10, and 12-16 are the sameas those in FIG. 1. Reference number 18 indicates a torque estimator forcalculating the output torque of the motor 1 based on the voltageinstruction values Vd** and Vq**, the detected current values Id and Iq,and the frequency instruction value ω1*.

[0130] The output signal τm{circumflex over ( )} is input to a functiongenerator 11 a 1 in the d-axis current instruction unit 11 a. Thefunction generator 11 a 1 calculates a correction gain Ga5 (0≦Ga5.≦1)based on the output signal τm{circumflex over ( )}.

[0131] In a multiplier 11 a 1, the increment ΔId* of the currentinstruction value is multiplied by the above Ga5. Further, Id** isobtained by adding Id* to the result of the multiplication, and the sumis output from the d-axis current instruction unit 11 a. Next, a torqueestimator 18 will be explained. The estimator 18 performs thecalculation shown by the equation (25) based on Vd** and Vq**, Id andIq, and ω1*.

τm{circumflex over ( )}=(Vd**·Id+Vq**·Iq)/ω1*  (25)

[0132] Id** which corresponds to the load torque is calculated accordingto the equation (26) using τm{circumflex over ( )} obtained by the aboveequation (25).

Id**=Id*+F(τm{circumflex over ( )})·ΔId*  (26),

[0133] provided that the output Ga5 of F(τm{circumflex over ( )}) is asfollows:

[0134] that is;

[0135] if |τm{circumflex over ( )}|=0, then Ga5 0, and

[0136] if |τm{circumflex over ( )}|≈0, then 0<Ga5≦1.

[0137] The value of Id according to the equation (26) is as follows:

[0138] without a load (|τm{circumflex over ( )}|=0), Id**=Id*, and

[0139] with a load (|τm{circumflex over ( )}|>0), Id**>Id*.

[0140] Thus, since Id** is corrected corresponding to the load torque,the operational efficiency in an operation with a light load can beincreased.

[0141] Although this control method is applied to the control apparatusshown in FIG. 1, if it is applied to the respective control apparatusesshown in FIGS. 5, 6, and 8, τm{circumflex over ( )} is calculated by theequation (27) using the voltage instruction reference value Vq* in placeof the voltage instruction value Vq** .

τm{circumflex over ( )}=(Vd**·Id+Vq*·Iq)/ω1*  (27)

[0142] By calculating Id** which changes corresponding to the loadtorque with the τm{circumflex over ( )} calculated according to theabove equation, the same operation and effects as those of the aboveembodiments can be achieved.

[0143] Moreover, although the estimated torque τm{circumflex over ( )}is calculated by using vd** and Vq**, and Id and Iq in this embodiment,it is possible to obtain τm{circumflex over ( )} according to theequation (28) using the estimated fluxes φ2d{circumflex over ( )} andφ2q{circumflex over ( )}.

τm{circumflex over ( )}=K1(φ2d{circumflex over ( )}·Iq−φ2q{circumflexover ( )}Id)  (28),

[0144] where K: a torque constant.

[0145] By calculating Id** which changes corresponding to the loadtorque with τm{circumflex over ( )} calculated according to the aboveequation, the same operation and effects as those of the aboveembodiments can be achieved.

[0146] Further, although the operational efficiency in an operation witha light load is improved by correcting Id, corresponding toτm{circumflex over ( )}, the same effects can be obtained using thecalculated slip frequency ωs** in place of τm{circumflex over ( )}.

[0147]FIG. 19 shows a schematic block diagram of the circuit compositionof a speed-control apparatus for an induction motor of this embodiment.This embodiment is an example of a control method for correcting Id** byusing ωs**, which is applied to the apparatus shown in FIG. 1.

[0148] In this figure, the units or devices 1-10, and 12-16 are the sameas those in FIG. 7. Reference number 11 b indicates a d-axis currentinstruction unit for calculating the gain Ga6 corrected with ωs**.

[0149] The calculated slip frequency ωs** is input to a functiongenerator 11 b 1 in the instruction unit 11 b. The function generator 11b 1 calculates the gain Ga6 (0≦Ga6≦1) to be corrected.

[0150] A multiplier 11 b 2 multiplies the gain Ga6 by the incrementΔId**. Further, Id** is obtained by adding the result of themultiplication to Id*, and the sum is output from the currentinstruction unit 11 b.

[0151] Next, the effects of the d-axis current instruction unit 11 bwhich is one of the main features of the present invention will beexplained below. The calculated slip frequency is proportional to thetorque.

[0152] Accordingly, if Id is controlled using ωs** in accordance withthe equation (29), the same effects as those of the above embodimentscan be obtained.

Id**=Id*+F(ωs**)·ΔId*  (29)

[0153] provided that the output Ga6 of F(ωs**) is as follows:

[0154] that is;

[0155] if |ωs**|=0, then Ga6 0, and

[0156] if |ωs**|>0, then 0<Ga6≦1.

[0157] If Id obtained by the equation (29) is used,

[0158] without a load (|ωs**|=0), Id**=Id*, and

[0159] with a load (|ωs**|>0), Id**>Id*.

[0160] Thus, since Id is corrected corresponding to the load torque(ωs**), it is apparent that the same operations and effects as the aboveembodiments can be obtained.

[0161] Although this control method is applied to the control apparatusshown in FIG. 1, if it is applied to the respective control apparatusesshown in FIGS. 5, 6, and 8, ωs{circumflex over ( )} is obtained by theequation (21) using the ratio of Δd to Vq*, and by correcting Id**corresponding to the sum ωs** of ωs{circumflex over ( )} and ωs*, thesame operations and effects as the above embodiments can be obtained.

[0162] In accordance with the present invention, it is possible toprovide an apparatus using a speed-control method for an inductionmotor, which does not cause a shortage in the torque in the speed rangenear zero.

We claim:
 1. A rotational speed-control apparatus to control aninduction motor, the apparatus comprising: a power-conversion unit: acurrent-control unit to control current output from the power-conversionunit; a measuring unit to measure an output frequency instruction valueinputted to the power-conversion unit; a comparator to compare themeasured output frequency instruction value with a predetermined value;wherein the comparator provides an output to the current-control unit tocontrol the current output from the power-conversion unit so as to belarger than a current available during ordinary no-load operation upon adetermination by the comparator that the measured output frequencyinstruction value is less than the predetermined value, the currentoutput from the power-conversion unit being independent of a torqueoutput of the motor.
 2. A rotational speed-control apparatus to controlan induction motor, the apparatus comprising: a power-conversion unit; acurrent-control unit to control current output from the power-conversionunit in accordance with a current instruction value in a d-axiscorresponding to a flux axis in a rotating-flux coordinate system; arotational speed instruction value measuring unit to measure arotational speed instruction value inputted to the power-conversionunit; an estimated rotational speed value measuring unit to measure anestimated rotational speed value; a comparator to compare the measuredrotational speed instruction value and measured estimated rotationalspeed value with a predetermined value; wherein the comparator providesan output to the current-control unit to control d-axis current so as tobe larger than a current available during an ordinary no-load operationupon a determination by the comparator that one of the measuredrotational speed instruction value and measured estimated rotationalspeed value is less than the predetermined value.
 3. A rotationalspeed-control apparatus to control an induction motor, the apparatuscomprising: a power-conversion unit: a current control unit to controlcurrent output from the power-conversion unit in accordance with acurrent instruction value in a q-axis in a rotating-flux coordinatesystem; a rotational speed instruction value measuring unit to measure arotational speed instruction value inputted to the power-conversionunit; an estimated rotational speed value measuring unit to measure anestimated rotational speed value; a comparator to compare the measuredrotational speed instruction value and measured estimated rotationalspeed value with a predetermined value; wherein the comparator providesan output to the current-control unit to control q-axis current so as tobe larger than a current available during an ordinary no-load operationupon a determination by the comparator that one of the measuredrotational speed instruction value and measured estimated rotationalspeed value is less than the predetermined value.
 4. A rotationalspeed-control apparatus to control an induction motor, the apparatuscomprising: a power-conversion unit; a current-control unit to controlcurrent output from the power-conversion unit in accordance with acurrent instruction value in a d-axis corresponding to a flux axis in arotating-flux coordinate system and a current instruction value in aq-axis in a rotating-flux coordinate system; a rotational speedinstruction value measuring unit to measure a rotational speedinstruction value; an estimated rotational speed value measuring unit tomeasure an estimated rotational speed value; a comparator to compare themeasured rotational speed instruction value and measured estimatedrotational speed value with a predetermined value; wherein thecomparator provides an output to the current control unit to controld-axis current so as to be a first predetermined value greater than acurrent available during ordinary no-load operation and controllingq-axis current so as to be less than a second predetermined value uponone of the measured rotational speed instruction value and measuredestimated rotational speed value being less than the predeterminedvalue.
 5. The apparatus according to claim 4, wherein the secondpredetermined value is equal to
 0. 6. A rotational speed-controlapparatus to control an induction motor, the apparatus comprising: apower-conversion unit; a current-control unit to control current outputfrom the power-conversion unit in accordance with one of a currentinstruction value in a d-axis corresponding to a flux axis in arotating-flux coordinate system and a current instruction value in aq-axis in a rotating-flux coordinate system; a rotational speedinstruction value measuring unit to measure a rotational speedinstruction value; an estimated rotational speed value measuring unit tomeasure an estimated rotational speed value; a comparator to compare themeasured rotational speed instruction value and measured estimatedrotational speed value with a predetermined value; a calculator tocalculate a frequency instruction value using the rotational speedinstruction value in place of the estimated rotational speed value uponone of the measured rotational speed instruction value and measuredestimated rotational speed value being less than the predeterminedvalue.
 7. A rotational speed-control apparatus to control an inductionmotor, the apparatus comprising: a power-conversion unit; acurrent-control unit to control current output from the power-conversionunit in accordance with a current instruction value in a d-axiscorresponding to a flux axis in a rotating-flux coordinate system; arotational speed instruction value measuring unit to measure arotational speed instruction value; an estimated rotational speed valuemeasuring unit to measure an estimated rotational speed value; acomparator to compare the measured rotational speed instruction valueand measured estimated rotational speed value with a predeterminedvalue; wherein the comparator provides an output to the current-controlunit to control d-axis current so as to be a predetermined value greaterthan a current available during ordinary no-load operation and furthercomprising a calculator to calculate a frequency instruction value usingthe rotational speed instruction value in place of the estimatedrotational speed value upon one of the measured rotational speedinstruction value and measured estimated rotational speed value beingless than the predetermined value.
 8. A rotational speed-controlapparatus to control an induction motor, the apparatus comprising: apower-conversion unit; a current-control unit to control current outputfrom the power-conversion unit in accordance with a current instructionvalue in a q-axis in a rotating-flux coordinate system; a rotationalspeed instruction value measuring unit to measure a rotational speedinstruction value; an estimated rotational speed value measuring unit tomeasure an estimated rotational speed value; a comparator to compare themeasured rotational speed instruction value and measured estimatedrotational speed value with a predetermined value; wherein thecomparator provides an output to the current-control unit to controlq-axis current so as to be greater than a predetermined value andfurther comprising a calculator to calculate a frequency instructionvalue using the rotational speed instruction value in place of theestimated rotational speed value upon one of the measured rotationalspeed instruction value and measured estimated rotational speed valuebeing less than the predetermined value.
 9. A rotational speed-controlapparatus to control an induction motor, the apparatus comprising: apower-conversion unit; a voltage-calculation unit to calculate an outputvoltage reference value for the power-conversion unit based on an outputfrequency instruction value to the power-conversion unit and afrequency-calculation unit to control a frequency output from thepower-conversion unit in accordance with a rotational speed instructionvalue; a rotational speed instruction value measuring unit to measure arotational speed instruction value; a comparator to compare the measuredrotational speed instruction value with a predetermined value; and acurrent control unit; wherein the comparator provides a signal to thecurrent-control unit to control d-axis current so as to be at apredetermined value larger than a current available during an ordinaryno-load operation upon the measured rotational speed instruction valuebeing less than the predetermined value.
 10. A rotational speed-controlapparatus to control an induction motor, the apparatus comprising: apower-conversion unit; a voltage-compilation unit to calculate an outputvoltage reference value for the power-conversion unit based on an outputfrequency instruction value to the power-conversion unit and afrequency-control unit to control a frequency output from thepower-conversion unit in accordance with a rotational speed instructionvalue; a rotational speed instruction value measuring unit to measure arotational speed instruction value; a comparator to compare the measuredrotational speed instruction value with a predetermined value; and acurrent control unit; wherein the comparator provides an output to thecurrent control unit to control q-axis current so as to be larger than apredetermined value upon the measured rotational speed instruction valuebeing less than the predetermined value.
 11. A rotational speed-controlapparatus to control an induction motor, the apparatus comprising: apower-conversion unit to control a frequency and voltage output from thepower-conversion unit in accordance with a rotational speed instructionvalue; an estimated slip frequency value calculator to calculate anestimated slip frequency value in accordance with the voltage outputfrom the power-conversion unit; a slip frequency value calculator tocalculate a slip frequency value in accordance with a current outputfrom the power-conversion unit; an adder to add the calculated estimatedslip frequency value to the calculated slip frequency value to produce aresultant sum; wherein the adder provides an output to thepower-conversion unit to control the frequency output of thepower-conversion unit in accordance with the resultant sum.
 12. Arotational speed-control apparatus to control an induction motor, theapparatus comprising: a power-conversion unit; a voltage-calculationunit to calculate an output voltage reference value for thepower-conversion unit in accordance with one of a pair of detected d andq-axis current values and a pair of d and q-axis current instructionvalues and in accordance with an output frequency instruction value tothe power-conversion unit and to control a frequency output from thepower-conversion unit in accordance with a rotational speed instructionvalue; an estimated slip frequency value calculator to calculate anestimated slip frequency value in accordance with the voltage outputfrom the power-conversion unit; a slip frequency value calculator tocalculate a slip frequency value in accordance with a current outputfrom the power-conversion unit; an adder to add the calculated estimatedslip frequency value to the calculated slip frequency value to produce aresultant sum; wherein the adder provides an output to thepower-conversion unit to control the frequency output of thepower-conversion unit in accordance with the resultant sum.
 13. Arotational speed-control apparatus to control an induction motor, theapparatus comprising: a power conversion unit and a current-control unitto control current output from the power-conversion unit and to controla frequency output from the power-conversion unit in accordance with oneof a rotational speed instruction value and an estimated rotationalspeed value; an estimated slip frequency value calculator to calculatean estimated slip frequency value in accordance with the voltage outputfrom the power-conversion unit; a slip frequency value calculator tocalculate a slip frequency value in accordance with a current outputfrom the power-conversion unit; an adder to add the calculated estimatedslip frequency value to the calculated slip frequency value to produce aresultant sum; wherein the adder provides an output to thepower-conversion unit to control the frequency output of thepower-conversion unit in accordance with the resultant sum.
 14. Theapparatus according to any one of claims 11-13, wherein the estimatedslip frequency value calculator calculates the estimated slip frequencyvalue in accordance with a ratio of a d-axis induced-electromotive forcevalue to a q-axis induced-electromotive force value.
 15. The apparatusaccording to any one of claims 11-13, wherein the estimated slipfrequency value calculator calculates the estimated slip frequency valuein accordance with one of a d-axis voltage instruction value and adetected d-axis voltage value, and one of a q-axis voltage instructionvalue and a detected q-axis voltage value.
 16. The apparatus accordingto any one of claims 11-13, wherein the estimated slip frequency valuecalculator calculates the estimated slip frequency value in accordancewith a ratio of a q-axis flux value to a d-axis flux value.
 17. Theapparatus according to any one of claims 11-13, wherein the estimatedslip frequency value calculator calculates the estimated slip frequencyvalue in accordance with a ratio of a q-axis flux value, obtained inaccordance with the value of the frequency output from thepower-conversion unit and one of a d-axis voltage instruction value anda detected d-axis voltage value, to a d-axis flux value, obtained inaccordance with the value of the frequency output of thepower-conversion unit and one of a q-axis voltage instruction value anda detected q-axis voltage value.
 18. The apparatus according to claim16, further comprising: a d-axis flux value calculator to calculate thed-axis flux value in accordance with a ratio of a value obtained bysubtracting a sum of a q-axis resistance voltage-decrease and a q-axisleakage-reactance voltage decrease from one of a q-axis voltageinstruction value and a detected q-axis voltage value, to the value ofthe output frequency; and a q-axis flux value calculator to calculatethe q-axis flux value in accordance with a ratio of a value obtained bysubtracting a difference between a d-axis resistance voltage-decreaseand a d-axis leakage-reactance voltage-decrease from one of a d-axisvoltage instruction value and a detected d-axis voltage value, to thevalue of the output frequency.
 19. The apparatus according to claim 16,further comprising: a d-axis flux value calculator to calculate thed-axis flux value by integrating a value obtained by subtracting adifference between a d-axis resistance voltage-decrease and a d-axisleakage-reactance voltage-decrease from one of a d-axis voltageinstruction value and a detected d-axis voltage value and furthersubtracting a d-axis induced-electromotive force from a result of thesubtraction; and a q-axis flux value calculator to calculate the q-axisflux value by integrating a value obtained by subtracting a differencebetween a q-axis resistance voltage-decrease and a q-axisleakage-reactance voltage-decrease from one of a q-axis voltageinstruction value and a detected q-axis voltage value and further addinga q-axis induced-electromotive force to a result of the subtraction. 20.The apparatus according to any one of claims 11-13, wherein the slipfrequency value calculator calculates the slip frequency value inaccordance with a ratio of one of the q-axis current instruction valueand the detected q-axis current value to a d-axis flux value.
 21. Arotational speed-control apparatus to control an induction motor, theapparatus comprising: a power-conversion unit to control a frequency andvoltage output from the power-conversion unit in accordance with arotational speed instruction value; a rotational speed instruction valuemeasuring unit to measure a rotational speed instruction value; acomparator to compare the measured rotational speed instruction valuewith a predetermined value; wherein the comparator provides an output tothe power-conversion unit to control current output from thepower-conversion unit so as to be at a predetermined value larger than acurrent available during an ordinary no-load operation, and to correctthe output current in correspondence with a value of a torque output ofthe motor upon the measured rotational speed instruction value beingless than the predetermined value.
 22. A rotational speed-controlapparatus to control a frequency output from a power-conversion unit foran induction motor in accordance with a rotational speed instructionvalue using a voltage-calculation unit to calculate an output voltagereference value for the power-conversion unit based on one of a pair ofdetected d and q-axis current values and a pair of d and q-axis currentinstruction values and based on a value of the frequency output from thepower-conversion unit, the apparatus comprising: a rotational speedinstruction value measuring unit to measure a rotational speedinstruction value; a comparator to compare the measured rotational speedinstruction value with a predetermined value; wherein the comparatorprovides an output to the power-conversion unit to control currentoutput from the power-conversion unit so as to be a predetermined valuelarger than a current available during an ordinary no-load operation,and to correct the output current in correspondence with a value of atorque output of the motor upon the measured rotational speedinstruction value being less than the predetermined value.
 23. Arotational speed-control apparatus to control a frequency output from apower-conversion unit for an induction motor in accordance with one of arotational speed instruction value and an estimated rotational speedvalue using current-control units for controlling current output fromthe power-conversion unit, the apparatus comprising: a rotational speedinstruction value measuring unit to measure a rotational speedinstruction value; an estimated rotational speed value measuring unitthe estimated rotational speed value; a comparator to compare themeasured rotational speed instruction value and the measured estimatedrotational speed value with a predetermined value; wherein thecomparator provides an output to the power-conversion unit to controlcurrent output from the power-conversion unit so as to be at apredetermined value larger then a current available during an ordinaryno-load operation, and to correct the output current in correspondencewith a value of a torque output of the motor upon one of the measuredrotational speed instruction value and measured estimated rotationalspeed value being less than the predetermined value.
 24. The apparatusaccording to any one of claims 21-23, further comprising a d-axiscurrent corrector to correct a value of d-axis current in correspondencewith the value of the torque output of the motor.
 25. The apparatusaccording to any one of claims 21-23, further comprising a q-axiscurrent corrector to correct a value of q-axis current in correspondencewith the value of the torque output of the motor.
 26. The apparatusaccording to any one of claims 21-23, further comprising a means forobtaining the value of the torque output based on one of a pair ofd-axis and q-axis voltage instruction values and a pair of detectedd-axis and q-axis voltage values and based on one of a pair of d-axisand q-axis current instruction values and a pair of detected d-axis andq-axis current values, the d-axis and q-axis being axes in arotating-flux coordinate system.
 27. The apparatus according to any oneof claims 21-23, further comprising a means for obtaining the value ofthe torque output based on one of a pair of d-axis and q-axis voltageinstruction values and a pair of detected d-axis and q-axis voltagevalues, the d-axis and q-axis flux values being calculated with thevalue of the frequency output from the power-conversion unit and basedon one of a pair of d-axis and q-axis current instruction values and apair of detected d-axis and q-axis current values in a rotatingcoordinate system, the d-axis and q-axis being axes in a rotating-fluxcoordinate system.
 28. A rotational speed-control apparatus to control afrequency and current output from a power-conversion unit for aninduction motor in accordance with a rotational speed instruction value,the apparatus comprising: a rotational speed instruction value measuringunit to measure the rotational speed instruction value; a comparator tocompare the measured rotational speed instruction value with apredetermined value; wherein the comparator provides an output to thepower-conversion unit to control current output from thepower-conversion unit so as to be at a predetermined value larger than acurrent available during an ordinary no-load operation and to correctthe value of the current in accordance with a value obtained by adding acalculated estimated slip frequency value based on a value of a voltageoutput from the power-conversion unit to a calculated slip frequencyvalue based on a value of current output from the power-conversion unitupon the measured rotational speed instruction value being less than thepredetermined value.
 29. A rotational speed-control apparatus to controla frequency output from a power-conversion unit for an induction motorin accordance with a rotational speed instruction value using avoltage-calculation unit to calculate an output voltage reference valuebased on one of a pair of d-axis and q-axis current instruction valuesand a pair of detected d-axis and q-axis values in a rotating-fluxcoordinate system and based on the value of the frequency output of thepower-conversion unit, the apparatus comprising: a rotational speedinstruction value measuring unit to measure the rotational speedinstruction value; a comparator to compare the measured rotational speedinstruction value with a predetermined value; wherein the comparatorprovides an output to the power-conversion unit to control currentoutput from the power-conversion unit so as to be at a predeterminedvalue larger than a current available during an ordinary no-loadoperation and to correct the value of the current in accordance with avalue obtained by adding a calculated estimated slip frequency valuebased on a value of a voltage output from the power-conversion unit to acalculated slip frequency value based on a value of current output fromthe power-conversion unit upon the measured rotational speed instructionvalue being less than the predetermined value.
 30. A rotationalspeed-control apparatus to control a frequency output from apower-conversion unit to drive an induction motor in accordance with oneof a rotational speed instruction value and an estimated rotationalspeed value using a current-control unit to control current output fromthe power-conversion unit, the apparatus comprising: a rotational speedinstruction value measuring unit to measure the rotational speedinstruction value; an estimated rotational speed value measuring unit tomeasure the estimated rotational speed value; a comparator to comparethe measured rotational speed instruction value and the measuredestimated rotational speed value with a predetermined value; wherein thecomparator provides an output to the power-conversion unit to controlcurrent output from the power-conversion unit so as to be apredetermined value larger than a current available during ordinaryno-load operation upon one of the measured rotational speed instructionvalue and the measured estimated rotational speed value being less thanthe predetermined value.
 31. The apparatus according to any one ofclaims 28-30, further comprising a means for correcting a value of thed-axis current in correspondence with a sum of the estimated slipfrequency value and the calculated slip frequency value.
 32. Theapparatus according to any one of claims 28-30, further comprising ameans for correcting a value of the q-axis current in correspondencewith a sum of the estimated slip frequency value and the calculated slipfrequency value.